Many types of equipment and industrial processes require accurate control of the flow of liquid and gaseous fluids over a broad range of fluid pressure and flow rates. It is particularly important that flow controllers for such equipment and processes be able to accurately meter fluid flow at low flow rates. One conventional type of flow rate controller utilizes a floating piston that moves within a valve chamber under fluid pressure to mate a valve pin earned by the piston with a valve seat. Such floating piston flow controllers are designed to provide a constant flow rate despite fluctuations in the pressure of the fluid being supplied to the controller. The theory of floating piston flow control devices is self set in U.S. Pat. No. 4,893,649 to Skoglund, the disclosure of which is hereby incorporated by reference. The Skoglund device includes a piston that carries a centrally mounted pin and also includes a radially offset longitudinal passage through the piston to permit fluid flow through the valve chamber. The pin mates with a valve seat that is adjustably positioned at one end of the valve chamber by an adjustable shaft. The shaft can be manually operated to move the valve seat relative to the piston for adjustment of flow control.
Devices such as that disclosed in the Skoglund '649 patent have certain disadvantages. Because both the piston and the valve seat assembly are slidably mounted within the valve assembly, seals are required on both the piston and the valve seat. The number of parts required increases the cost of manufacture and assembly of the valves, and each seal represents a potential for failure that could adversely affect the performance of the valve. Additionally, because the flow passage through the piston is radially offset, there is a potential for the piston, and thus the valve pin, to become cocked as the result of a pressure imbalance on the piston. This is particularly true when cavitation occurs during shut-off of flow. When the pin is cocked relative to the valve seat, flow shut-off is incomplete and leakage through the valve will occur.
Another drawback of conventional floating piston valve controllers such as that disclosed by Skoglund '649 is that the amount of torque required to adjust the valve by repositioning the valve seat increases proportionally to fluid pressure within the valve. This is because the fluid pressure is operating in the valve seat components that must be rotated. As a result, adjustment can be difficult to accomplish at high pressures.